Intravaginal placebo sponges affect negatively the conception rate in sheep

•The use of intravaginal sponge in ewes for oestrous synchronization induces vaginitis.•The intravaginal sponge is responsible itself for an important reduction in conception rate.•The applications of sponges, even without hormonal content, decrease the fertility.

Intravaginal sponges (IS) impregnated with progestagens are commonly used for oestrous synchronization in ewes. However, the use of IS is a predisposing factor for vaginal contamination that can affect negatively fertility. The objective of this work was to determine if the use of IS negatively affects the fertility. The experiment was conducted on a commercial farm (Uruguay), during March-May (mid-breeding season) with 575 mature Australian Merino ewes. Oestrous was recorded daily with vasectomized rams for 5 d in ewes that received a single dose of a PGF2alpha analogue. Two days after oestrous detection, females were randomly allocated to three groups: Group MAP (n = 200) received an intravaginal polyurethane sponge impregnated with 60 mg of medroxyprogesterone acetate (MAP) for 13 days. At the same time, sponges without MAP were inserted in 156 ewes (ISG Group), and remained in situ also for 13 days. Other 213 ewes (Group CG) remained as a control of the fertility of the spontaneous oestrus, without treatment. Ewes presenting oestrous were inseminated 12 h later. Conception rates were determined by rectal ultrasound 30 days after oestrus. Conception rate of the CG group was greater than that of MAP and ISG groups (CG: 55.4% vs MAP: 41.5% and ISG: 34.6%; p = 0.0002), without differences between the last two. We conclude that the IS is responsible itself for a significant reduction in conception rate in oestrous synchronization treatments in sheep.